Molakule did a nice write up on this awhile back,find his member number and do a search
In the old days,say a guy with 20 wt oil during winter in his car or truck that was near time to change the oil and just wanted either to pull a load or make a fairly long high speed road trip and drain when he returned it was effective in that sense,we must differntiate the STP's as the 4 cylinder treatment is somewhat thinner and contains more that just a thick bright stock and Zinc,Valvoline has a simaler product that has a graph showing it is superior to STP in oxidation ect,it should not be used in the differential. I use the 4cylinder additive to aid in breaking in cams with high spring pressures and hi intensity lobe designs"especially the unsymetrical lobes"
Oddly,Lucas Oil Treatment says much about using it in the crankcase,I think imo it would be better used in a gear box but I would not even do that imo. Only in a oil burning engine would I use any of these I have spoke about.
There are only two additves that are worth using imo and that is the thin Synergyn additive and the Shaeffer 132 Moly,,but would only use these in certain circumstances. I make my oil do it's job by it's self on daily drivers,if it needs some help,maybe a better oil or the same oil in a different viscosity that is up to the task should be used